The most hated woman in England was imprisoned in Holloway in the second world war as a danger to the king's realm. Diana Mosley, who has died in Paris aged 93, was that "famous awful person" - her words - scandalous beauty, friend to Adolf Hitler and wife of the leader of the British Union of Fascists (BUF), Sir Oswald Mosley.

And a Mitford girl, the third of six daughters of Lord Redesdale. Her elder sisters were Nancy, the novelist, and Pam, the least well-known. Younger than her were the pro-Hitler Unity, the communist Jessica and Deborah, who is the Duchess of Devonshire. Her elder brother Tom was killed in Burma in 1945.

Diana Freeman-Mitford, known as Nardy, Corduroy and Honks, had what passed for a normal childhood in that household, Asthall Manor in Oxfordshire (an appendectomy on a spare-bedroom table, side-saddle hunts with the Heythrop hounds) before first revealing her looks and revelling in their power on visits to Paris, although she was gated for months after the discovery of a diary entry about going to a cinema with a boy .

The only exit from chaperones and chilblains was marriage. Diana of the sapphire eyes was rated more perfect than Botticelli's Venus and attracted Bryan Guinness, heir to the brewing fortune, as soon as she was out in society. They were engaged secretly and Diana sulked until her parents approved the marriage. As she fretted over her bridal veil, Nancy told Diana, as she always had, "Nobody will be looking at you". But they were: it was the wedding of the year of 1929. Her mother-in-law led the Guinness household's amazement at Diana's ability to fry eggs - "I've never heard of such a thing, it's too clever". Not that Diana needed even that rudimentary culinary level, as the couple had £20,000 a year, a London house, Hampshire estate, Dublin home and so on.

A court of smitten men attended Diana in her progressions round the properties, even during her pregnancies with sons Jonathan and Desmond; Evelyn Waugh was the most passionate ("her beauty ran through the room like a peal of bells") and dedicated to her Vile Bodies, his novel inspired by a party the couple had thrown. The Guinnesses were too late and not nearly silly enough to be Bright Young Things, but knew all of those whom Diana called "originals" in the arts.

So Diana was a Cecil Beaton-photographed goddess at a summer ball in her Chelsea house in 1932, dancing until dawn "in all the diamonds I could lay hands on". Oswald Mosley declared his love that night. He had charisma and sexual avidity, later associated with showbiz, but then invested in public life: he had been a Conservative and Labour politician and, rejecting or rejected by both, founded his New Party in 1931 and soon after decided that fascism was the only answer to global collapse after the 1929 crash and set up the BUF in 1932.

Diana was intrigued by his ideas - "I followed him politically absolutely blindly" - and in love: "I knew it would never end except with death." They briefly encountered each other at parties; they were indiscreet among aristo-Brits holidaying on the Venice Lido. Against family advice, Diana left her husband, but Mosley would not part from his wife, Cynthia "Cimmie" Curzon, daughter of a former Viceroy of India.

Gentlemanly Guinness offered the requisite fake evidence of his "infidelity" for a divorce and Diana moved with her children to Belgravia, where (because a pending divorce could be denied if the King's Proctor found evidence of collusion), Mosley visited stealthily. She was a social pariah; he was in furious dispute with his wife, who collapsed with a perforated appendix. Mosley went straight from Cimmie's side to Diana: Cimmie died, unresisting, of peritonitis.

Mosley, guilty and grieving, began yet another affair; Diana fled to Bavaria with her sister Unity, then 19, a member of the BUF. A society acquaintance gave them tickets for the first Nazi Nuremberg rally, but wouldn't introduce them to Hitler, for all their blonde Aryan looks, because they wore too much lipstick (their father complained about that, too). They returned the next year to Hitler-watch - Unity stalked the Führer until she struck up a friendship, and introduced Diana. They were his guests at the 1935 rally; next year, he sent a Mercedes to chauffeur Diana to the Olympic Games in Berlin. Both sisters passed far beyond mere Vogue fascism - the social attitude that thought the Reich style too thrillingly modern.

Diana continued as Mosley's public mistress for three years, tolerant of his infidelities; she arrived by seaplane to join him on Capri, her face bandaged after plastic surgery to repair car-crash damage, only to have him decamp with a rival. When Diana got pregnant, her illegal abortion sobered the relationship. They rented a peculiar country house, Wootton Lodge, together.

Mosley had restyled the blackshirted BUF ("the shirts cost about 2s11d," said Diana) in the militaristic German and Italian manner, but a violent rally in 1934 and accentuated anti-semitism had forced the organisation out of mainstream politics. The BBC controlled British radio and Mosley realised there was potential profit to fund the BUF in a German-based commercial station transmitting nonstop popular music to Britain - Radio Luxembourg had made a fortune. Diana intended to sell cosmetics over the airwaves, and offer the Third Reich a share of hard-currency profits.

So from 1936 to 1939, Diana used her Nazi entrée to push Air Time Ltd, keeping Mosley free of direct contact with the deal. She took tea with Hitler by the fire in a German governmental HQ, and he asked her about her cousin Winston Churchill. Winston quizzed her about Herr Hitler. Diana married Mosley secretly, by special Reich permission at the family home of the Nazi propaganda chief Joseph Goebbels in Berlin in 1936, with Hitler as guest - he gave her a photograph of himself in an eagle-topped frame, which she deposited in a country branch of Drummonds bank at the outbreak of war ("I've got the little chit somewhere," she said at 90).

Their first son, Alexander, was born in 1938, and they made the marriage public. After the German takeover of Austria, Diana was informed "you have your wavelength and a very nice one" and there were plans for a transmitter on a North Sea island. But in August 1939, Hitler told Diana over lunch that war was inevitable, and when it was declared, the project collapsed. The Mosleys retreated to a blacked-out Wootton, then to a farm Cimmie had left him.

Mosley offered the services of the BUF to the British government, but he was loathed as a potential collaborator should the Nazis invade; part of the Labour party's price for joining a coalition government may have been his internment in Brixton prison in 1940. Diana, recently delivered of her son Max, was watched closely. Her ex-father-in-law and sister Nancy denounced her as a "sinister woman".

A month after Mosley's detention, she was told to pack a few clothes, hand Max - whom she was breastfeeding - and his brother over to relatives. She entered the filthy dark of Holloway Prison's F-wing, its plumbing fractured by bombing, its food contaminated. Churchill, as prime minister, directed that Diana could bathe daily, but water supplies were so low that ablutions were never more than weekly. She ordered bottles of "grocer's port" through her Harrods account, and Mosley sent her a whole Stilton; for months she subsisted on a daily glass and slice of cheese. A newspaper fantasised about Mosley's life inside, imagining a butler and unrationed viands; he sued for libel and the couple saw each other for moments at court. The damages paid for a rough fur coat, under which she slept.

Churchill granted her request that the Mosleys be imprisoned together in a small house inside Holloway walls. The couple had quiet, clean sex offenders as domestics; they raised and cooked vegetables, aubergines and fraises des bois besides cabbages, listened to records, read Racine, sunbathed in their tiny yard. The prison priest told the warden: "It's the Garden of Eden out there, Lady Mosley in her little knickers." When they were released in 1943, 20,000 people petitioned and protested against the decision. Exiled from London and with their other properties requisitioned, they grew food at Cimmie's farm, and tutored their sons at home.

Postwar, the Mosleys set up Euphorion Books to publish his anathemised writing, although printers snubbed them. For six years she edited the far-right magazine The European. Their diminished future was mostly outside England. They were refused passports, but Mosley stood by his rights under Magna Carta and bought a 60-ton ketch to sail to Lisbon in 1949 (the day before departure, passports arrived in the post). They cruised the Mediterranean, then settled in a bishops' palace in County Galway, Ireland, which burned down two years later. Their last home together was La Temple de la Gloire, a dolls-house pavilion in the Paris suburbs, with the Duke and Duchess of Windsor for neighbours.

Mosley reorganised the BUF as the Union Movement post-war and ran for parliament opposing non-white immigration in 1959. "Immigration has been a tragedy," Diana said, "any number should have been allowed in to go to the universities and learn to be doctors and one thing and another - but not to settle". Their rhetoric, if not their prejudices, seemed archaic.

He died in 1970 and she mourned him the rest of her life, and in her 1977 memoirs, A Life of Contrasts, infuriating by her lack of repentance. "But I didn't love Hitler any more than I did Winston. I can't regret it, it was so interesting," she said whenever possible, even on Desert Island Discs (a contentious broadcast and not just because she chose Wagner's Ride of the Valkyrie). She claimed that Mosley had never been anti-semitic, but she was inclined to such statements as, of the Jews, "maybe they could have gone somewhere like Uganda - very empty and lovely climate". That, in an awful way, is the authentic Mitford voice.

She is survived by the two sons of her first marriage and the two sons of her second.